How does one create a widget for an article?

You should be able to use the Download Button plugin to do this with a Text widget.

The plugin claims to let you create a Download button using a shortcode, so you could put some of the article text and the button inside a text widget.

You will also need to activate shortcodes inside widgets. Here is an article describing how to do that:

https://digwp.com/2010/03/shortcodes-in-widgets/